Britain's Secretary of State for Foreign and Commonwealth Affairs Boris Johnson arrives at 10 Downing Street in London, Britain, July 3, 2018. REUTERS

Foreign Secretary Boris Johnson quit on Monday over Prime Minister Theresa May's plans to leave the European Union, the second resignation in a day leaving the British leader's Brexit plans all but in tatters.

After a day when the foreign secretary cancelled meetings for crisis talks at his official residence in central London, Johnson decided to walk from his job - just hours after May's Brexit minister David Davis did the same in protest at her plans.

The two resignations leave May badly exposed at the top of a government unable to unite over Britain's biggest foreign and trading policy shift in almost half a decade.

It also puts a question mark over whether the leader will try to weather it and stand firm in her commitment to pursue a "business friendly" Brexit, or will be faced with more resignations and calls to quit herself.

"This afternoon, the Prime Minister accepted the resignation of Boris Johnson as Foreign Secretary," May's spokesman said in a statement. "His replacement will be announced shortly. The Prime Minister thanks Boris for his work."

The departures raise the stakes for May, who secured a hard-won agreement with her deeply divided cabinet of ministers on Friday to keep the closest possible trading ties with the EU.

Many eurosceptics are angry, saying the agreed strategy betrays her promise for a clean break with the EU, raising the prospect that some could try to unseat her.